Urea-formaldehyde and phenol-formaldehyde adhesives, both of which are derived from petroleum, have been widely used in the fabrication of interior plywood and particle board for decades. These synthetic adhesives are cheap, easy to use, have long pot lives, and very good adhesive performance. However, formaldehyde emissions from these adhesives pose potential hazards to the environment and to human health. Thus, the U.S. has enacted a law that effectively implements the stringent formaldehyde emission levels set by the California Air Resource Board [1] as a national standard. The Formaldehyde Standards for Composite Wood Products Act (15 U.S.C. Chapter 53, Sub-Chapter VI) went into effect nationwide in the United States on Jan. 1, 2013. While there are certain exemptions and phase-out periods included in the law, there is a critical, regulation-driven need to develop adhesives that perform at least as well as urea-formaldehyde and phenol-formaldehyde adhesives, but which do not emit formaldehyde or any other dangerous gases. An additional benefit would be if such an adhesive could be derived from renewable resources, rather than from fossil fuels.
Soy protein isolate (SPI) has been investigated for use as a wood adhesive. A host of modifications have been made to SPI in attempts to improve its adhesive qualities. For example, SPI has been modified to introduce moieties including phenol [2, 3], thiol[4], maleyl [5], amino [6], and hydroxyl[7, 8]. These efforts have not yielded encouraging results. Use of the cross-linking agent polyamidoamine-epichlorohydrin (PAE) as an additive to SPI has been shown to produce a soy flour adhesive with acceptable water resistance in plywood [9, 10]. The improved water resistance of soy flour-PAE adhesive is attributable entirely to the PAE and reaches a maximum value at about 5 wt % PAE (20 wt % of total dry solids) in the formulation [11]. Notably, however, PAE is also derived from petroleum.
Thus, the problem remains that these prior art modifications all rely upon petroleum-based solvents and chemicals to improve the adhesive performance of SPI-based and soy flour-based adhesives. Moreover, it is unknown whether these petroleum-based additives also outgas harmful vapors over time. There thus remains a long-felt and unmet need for a simpler, non-petroleum-based “green” chemical modification strategy to transform soy flour into a form that will yield an adhesive with acceptable dry binding strength and water resistance.